This application claims the priority of German Patent Application Serial No. 101 10 921.0, filed Mar. 7, 2001, the subject matter of which is incorporated herein by reference.
The present invention relates, in general, to a spreader dowel, and more particularly to a dowel of a type having a sleeve formed with an internal thread for threaded engagement of a screw and having longitude slots on one end for providing an expansion zone which can be widened, when a spreader piece, connected to the screw and having a conical portion, is engaged therein.
It is to be understood by persons skilled in the art that the term xe2x80x9cspreader dowelxe2x80x9d is used in the following description in a generic sense and refers to any expansible fastener, which generally follows the concepts outlined here, including those connectors also referred to as xe2x80x9cexpansible anchorsxe2x80x9d or xe2x80x9cexpansible plugsxe2x80x9d.
Dowels of this type are widely used for many applications. For example, dowels can be used to secure fittings or the like in hollow sections. German publication DE-OS 2157961 describes a spreader dowel which has a sleeve provided with longitude slots in the outer surface area which faces the entry side for the screw, whereas the opposite end zone of the sleeve includes an internal thread. The screw is intended to extend through an object, such as a fitting, to be fastened and is formed integrally with a spreader piece with a conical prolongation. As the screw is tightened, the sleeve is forced against the hollow section wall, with the fitting resting on the other side thereof, while the segments, formed by the longitude slots, are spread at a same time by the inserted spreader piece. A clamping of the sleeve and support of the end face thereof by the section wall results in a fastening of the fitting in axial direction of the screw and spreader dowel.
This conventional spreader dowel suffers shortcomings because the spreader dowel is only able to absorb tensile forces. Bending forces that may be encountered cannot be absorbed by the spreader dowel. As a consequence, in particular when the fitting is an object such as a door handle or window handle and subject to strain during repeated use, the resultant changing loads lead ultimately to a loosening of the connection. Moreover, in those applications that involve the use of this conventional spreader dowel for fastening heavy objects, the holding capability of the spreader dowel is not exploited optimally because as a result of its construction only relatively small areas of the sleeve can be brought in contact with surrounding material. Also, problems have been encountered, when using the conventional spreader dowel to fasten objects such as fittings, as a result of the conjoint rotation of the sleeve with the screw, as the screw is tightened. The reason is the absence of any lateral holding of the sleeve, which is restrained only in the area of the end face where the screw is inserted. Naturally, this complicates installation and results in relatively high mounting costs.
It would therefore be desirable and advantageous to provide an improved spreader dowel which obviates prior art shortcomings and is easier to mount, while yet is able to withstand higher loads.
According to one aspect of the present invention, a spreader dowel includes a sleeve having an area, which is formed with an internal first thread for engagement by a screw, and including first longitude slots on one end for defining an expansion zone, and a spreader piece having a conical portion connected to the screw and widening the expansion zone, when the screw is inserted, wherein the sleeve has second longitude slots, extending inwardly from an end face on the other sleeve end, and includes in a region of the second longitude slots toward the end face a zone which is formed with an internal second thread defined by a thread diameter which is smaller than a thread diameter of the screw while having a same pitch as the thread of the screw, or, as an alternative, is threadless with a clear diameter which is smaller than an outer diameter of the screw.
A spreader dowel according to the present invention has many advantages. In conjunction with a fastening of an object, such as a fitting, to one wall of a hollow section, the screw can be tightened much easier because the sleeve of the spreader dowel can be inserted first and held by the second wall of the hollow section, i.e. the wall that is opposite the wall on the entry side for the screw, until the screw is guided by the object into the sleeve. The bore in the second wall of the hollow section has a diameter which corresponds to the diameter of the first wall. As it is received in the second section wall, the spreader dowel according to the present invention is supported twofold in conjunction with the inserted screw, thereby significantly improving the load-carrying capability. In addition, the novel and inventive spreader dowel is able to absorb bending forces so that the fastening strength of the attached object is significantly improved.
When mounting the spreader dowel, the area immediately confronting the engagement side is widened first so as to effect a press-fit against the surrounding wall. Thus, the spreader dowel is secured in axial direction and restrained from slipping. Only during further tightening of the screw does the spreader piece engage, thereby expanding the area of the first longitude slots and securing it in place through abutment against the surrounding wall.
According to another feature of the present invention, the first longitude slots and the second longitude slots are arranged in offset relationship. In this way, the longitude slots can be sized sufficiently long to overlap, thereby realizing a press-fit of the sleeve nearly over the entire length thereof, especially when the spreader dowel is inserted in a bore that fully surrounds the sleeve. As a consequence, the pressure force is increased by a multiple compared to prior art spreader dowels of the so-called heavy-duty type, so that the load-carrying capability is also enhanced.
According to another feature of the present invention, a threaded zone is provided to immediately follow the area of the internal first thread with same nominal diameter as the screw toward the free end, i.e. the end distal to the engagement side, whereby the threaded zone has a nominal diameter which is smaller than the nominal diameter of the screw so that the expansion is realized only when the screw is engaged in this zone. Hereby, the nominal diameter of the threaded zone may correspond to the minor diameter of the screw thread.
The manufacturing process of the spreader dowel according to the present invention may be further simplified, when the zone following the area of the internal first thread is smooth and thus threadless, whereby the clear diameter of the zone is slightly smaller than the outer diameter of the screw or the nominal diameter of the thread. Suitably, the clear diameter of the smooth zone corresponds to the minor diameter of the thread.
According to another feature of the present invention, the spreader piece is held non-rotatably in the sleeve. This can be realized by providing the spreader piece with a nose for engagement in a complementary recess of the sleeve or in one of the first longitude slots.